Vapor vacuum pump of the double jet type



P. ALEXANDER'ETAL.

VAPOR'VACUUM-PUMP OF THE'DOUBLE JET TYPE Filed May 26, 1945 PatentedJuly 16, 1946 UNITED STATES PATENT OFFICE Paul Alexander, Berkhamsted,and Cecil Whiley, London, England Application May 26, 1945, Serial No.596,035 In Great Britain April 21, 1945 1 claim. l

rhis invention relates to vapor vacuum pumps of the double jet type.

Mercury is the liquid usually vaporized in Vapor pumps but otherliquids, such as oils, are employed. In pumps of this type mercury vaporfrom a mercury boiler ascends a central tube, the upper jet being xed atthe top of the tube, while the lower jet is xed about halfway up thetube; both jets are supplied with vapor from the central tube, and thevapor issues from the jets into the annular space between the centraltube and the outer tube constituting the body of the pump. Theefficiency of the pump for a given vacuum depends largely on thedensity' of the vapor issuing from the two jets and on the downwardvelocity of the molecules in this vapor.

Now it has been found that, in the described arrangement in which thetwo jets are supplied with vapor from the same source, it is generallyimpracticable to design the two jets so as to produce vapor jets withthe desired density and velocity. There is necessarily a fall in vaporpressure from the bottom to the top of the central tube to produce aflow up the tube, and the pressure at one of the jets depends on therate of ilow through the other jet. The two jets therefore areinterdependent.

According to the invention, the two jets are supplied with vapor throughtwo non-communicating passages from two non-communicating evaporatingsurfaces of two communicating masses of liquid. The upper jet may besupplied with vapor through a central tube descending below the level ofthe liquid in the boiler and separating `the liquid in the boiler into acentral portion within the tube and an annular portion outside the tube,and by the lower jet being supplied with vapor from the annular portionof the liquid through a tube between the central tube and the pumpingspace, and holes in the central tube below the level of the liquidgiving communication between the two portions. rPhe tube through whichvapor is supplied to the lower jet may be connected with the outer wallof the pumping space through a liquid seal, through which liquidcondensed in the pumping space returns to the annular portion of theliquid in the boiler.

In the accompanying drawing, which shows in a single igure by way ofexample a form of pump embodying the invention in vertical section, amercury boiler I, containing mercury normally up to the level of thedotted line 2, with heater 3, supports the central vapor tube 4 and theouter tube 5 forming the body of the pump, which terminates above in theinlet 6 of the pump. The upper annular jet l, with annular orice 3 isfixed on the top of the central tube 4. The vapor from the mercury inthe boiler, passing up the tube 4 enters the jet 'l through holes I0 inthe central body of the jet and issues from the orifice 8 into thepumping space II between the tubes 4 and 5.

The lower jet has its outer member I2 xed to the tube 4, While its innermember i3 is fixed to the top of a tube I4 attached to the outer tube 5at I5.

The mercury in the boiler is separated into two portions, a. centralportion inside the tube 4 and an annular portion between the tubes 4 and5.V The two portions are in communication lthlrough the holes I9 in thetube 4 below the mercury level 2, and the two portions are heatedtogether by an electric heater in the space 3. Mercury evaporated fromthe annular portion ascends into the space I6 between the tubes Il andI4 and issues from the lower jets I I3. The evaporation from thisportion is independent of that from the central portion, and the vaporpressure in the tube 4 leading to the upper jet may diier from that inthe space i8 leading to the lower jet, any diierence in vapor pressurebeing compensated by a difference of level of the mercury inside andoutside the tube 4.

The upper jet, therefore, operates independently of the lower jet, andthe quantity of vapor issuing from each jet depends only on thedimensions of the jet and the temperature at which the mercury ismaintained.

The outer tube 5 is surrounded as is customary by a water jacket 25, andthe outlet 2l of the pump leads from the bottom of the space li to thebacking pump. The mercury vapor from the two jets is condensed by thecooled wall 5 and falls into the space 22 between the tubes I4 and 5. Amercury seal is formed at this point by the channel 23, and thecondensed mercury passes through holes 24 in the tube I and, overflowingthe channel 23, returns to the boiler.

As shown in the drawing, the outer tube 5 pref erably has parallel wallsat its upper part surrounding the jet l, the annular space between thetube 5 and jet I of width A determining in large measure the rate ofevacuation of the pump.

As described and claimed in co-pending application No. 596,034 the tube5 diminishes in diameter by the inclusion of a conical wall just belowthe jet l, which conical wall joins up with the parallel walls of thelower part of the tube 5, so that the tube 5 diminishes in diameter at apoint near jet orifice level so that the length of vapor jet measuredalong a line B with angle 15 degrees from the general pumping direction,indicated by the line C, does not exceed 2.6 times the width A of theannular space at jet orice level.

Further as described and claimed in co-pending application No. 596,036 awater jacket 25 may surround, but not touch, the tube 4, for a lengthjust below the jet orice 3Y downwards, the jacket having inlet andoutlet pipes 26 by which it is continuously supplied with cold water.

As fully explained in the aforesaid co-pending application No. 596,036,the eciency of the pump is further increased by the employment of thisinner jacket since vapor molecules are thereby prevented from strikingthe hot tube A and achieving a random velocity which enables suchmolecules to collide with molecules issuing from the jet 8 and diminishthe downward component of their velocity, 't

4 We claim: A vapor vacuum pump comprising a boiler for the vapor, acentral tube depending into said boiler and having apertures in itsdepending part,

' an outer tube Vforming with said central tube an annular space in saidboiler, an intermediate tube between said central and outer tubesforming inner and outer annular spaces respectively therebetween, upperand lower annular jets supplied with vapor from the boiler respectivelythrough the central tube and the inner annular space, and

